My invention relates to a phase locked loop, and particularly to an acquisition circuit for a phase locked loop, and method pertaining thereto.
Phase locked loops are used extensively in electronic and radio apparatus to provide a signal whose frequency can be easily varied and whose stability can be maintained by a reference oscillator.
Accordingly, a primary and general object of my invention is to provide a new and improved acquisition circuit for a phase locked loop.
Another object of my invention is to provide a new and improved acquisition circuit for a phase locked loop that can reliably acquire phase lock (sometimes called pull-in) over a relatively wide band of frequencies, and maintain that acquired phase lock.
While acquisition circuits have been previously provided for phase locked loops, such acquisition circuits have not been as simple and reliable in operation as the state of the art and present electronic and radio apparatus require.
Accordingly, another object of my invention is to provide a new and improved acquisition circuit for a phase locked loop that is relatively simple and reliable in achieving acquisition.
Another object of my invention is to provide a new acquisition circuit for a phase locked loop that can reliably sweep a voltage controlled oscillator (VCO) over a wide range of frequencies to achieve phase lock.
Another object of my invention is to provide a new and improved method for achieving and maintaining acquisition in a phase locked loop.
In some phase locked loops, a mixer is used in place of or in addition to a frequency divider or multiplier to cause the voltage controlled oscillator (VCO) to operate at the desired frequency relative to a local oscillator frequency. As the VCO is swept, it can produce a frequency which is the image of the desired frequency. (The image frequency is the frequency on one side of and separated from the local oscillator frequency by the same amount that the desired frequency is on the other side of and separated from the local oscillator frequency.) When the image frequency is produced, the mixer produces a frequency which appears correct to the phase locked loop, and causes the phase locked loop to try to lock on that incorrect image frequency. This results in an unstable condition. Or, because of the mixing or frequency dividing sometimes used in the phase locked loop, phase lock may incorrectly occur on a harmonic frequency.
Accordingly, another object of my invention is to provide a new and improved acquisition circuit which prevents a phase locked loop from locking or attempting to lock on the improper image or harmonic frequency.
In some applications where a phase locked loop is used, power consumption is quite important. This is particularly true of battery powered radio transmitter and receiver equipment, such as a hand-held personal transceiver.
Accordingly, another object of my invention is to provide a new and improved acquisition circuit for a phase locked loop which can operate relatively quickly when power is applied for transmitting and receiving.
Another object of my invention is to provide a new and improved acquisition circuit which has means for preventing a transmitter from operating until acquisition and phase lock are obtained.
Another object of my invention is to provide a new and improved acquisition circuit which has means for preventing acquisition sweep, with resultant radio frequency interference, if the acquisition circuit is operating improperly.